gpiolib: Change bitmap allocation to kmalloc_array

We don't need to clear out these bits when we set them immediately
after. Use kmalloc_array() to skip clearing the bits.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Tested-by: Timur Tabi <timur@codeaurora.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>